oa The Bark without a Dog - Magnetic Anomalies over Holes in a Volcanic Sheet in the greater McArthur Basin, NT

Linear strings of circular negative magnetic anomalies in the greater McArthur Basin of the Northern Territory are interpreted as due to holes in an underlying sheet of Kalkarindji flood basalts. Individual anomaly inversion results provide an estimate of the diameter, depth to top, and depth extent of holes in the volcanic sheet. The effective magnetization of the hole is its contrast against the more strongly magnetised sheet. Estimated magnetization contrast values are mostly rotated from a direction antiparallel to the local geomagnetic field, which we interpret as due to the contribution of remanent magnetization within the sheet. We support interpretation of the anomalies as due to holes in the sheet by comparing them with the magnetic expression of distant sheet edges. The linear arrangement of the anomalies is believed to arise from fractures in the sheet, suggesting that the holes developed after the sheet was emplaced, most probably by local escape of fluids, which altered the sheet and destroyed its magnetization.